Replacing an Old Hot Water Tank With an Instant System
The swap most people ask about when the old tank starts leaking.
You can replace a hot water tank with an instant system in most Australian homes, and the job is simple if you swap like for like: a gas tank becomes a gas continuous flow unit, and an electric tank becomes an instant electric unit only if the house has 3-phase power. That is the whole trick to replace hot water tank with instant. Gas to gas is usually a half day job. Electric to electric on single phase power is where people get caught, because single phase cannot run a whole house instant heater.
Want your exact size? Our instant hot water sizing calculator works out the kW or litres a minute you need in winter, checks single or 3-phase power, then shows the units that fit.
Which replacement suits your old tank
The replacement for an old hot water tank is decided by what feeds the tank now: gas, 3-phase power or single phase power.
| Your old tank | Instant replacement | Difficulty |
|---|---|---|
| Gas storage, mains natural gas | Gas continuous flow, Midea M20 or Midea M26 | Easy, if the gas line is big enough |
| Gas storage, indoor cupboard | Midea M26 internal, flued to outside | Medium, needs a proper flue |
| Electric storage, 3-phase house | Instant electric, Stiebel Eltron DHE 27 or DEL 27 Plus | Medium, new dedicated circuit |
| Electric storage, single phase house | Not a whole house instant unit. Gas or a heat pump instead | Hard, or the wrong job |
| Small electric tank under a sink | Point of use, Stiebel Eltron DHC-E 8/50 | Easy |
Check which power supply you have before anything else. Our explainer on single phase vs 3-phase instant hot water shows how to tell from your switchboard.
Swapping a gas tank for gas continuous flow
Swapping a gas storage tank for a gas continuous flow unit is the easiest tank to instant conversion in Australia, because the gas, water and drain are already at the spot.
The catch is gas volume. A continuous flow burner fires hard for a few minutes, where a tank burner sips gas all day. So the burner on an instant unit is much bigger than the one in your old tank. Your gas fitter has to check the meter and the pipe run can feed it. A short run from a newer meter is usually fine. A long, skinny pipe from an old meter sometimes is not, and upsizing it is the main cost blowout on these jobs.
The other things that change:
- Power point. The Midea M Series needs a standard power point near the unit to run its controls and fan. Old gas tanks often had none.
- Location. Outdoor units go on an outside wall. If the old tank lived in a laundry cupboard, either move the hot water to the outside wall or use the internal model with a flue.
- Size. 20 litres a minute suits Brisbane, Perth and Sydney. Melbourne, Adelaide and anywhere colder wants 26 litres a minute, because colder inlet water takes more heating.
The Midea M20 is $749 and the Midea M26 is $849, unit only, both with a 5 year residential warranty.
Prices current as of October 2026, AUD including GST, unit only. Installation not included.
Swapping an electric tank for instant electric
Swapping an electric storage tank for instant electric only works for a whole house if the property has 3-phase power.
Here is why. A typical electric tank heats slowly with an element of a few kW, often on an off-peak circuit overnight. An instant heater has to do the same job in real time while you stand under the shower. One standard shower at 40°C in winter needs about 15 kW in Brisbane, 17 in Sydney, 19 in Melbourne and 21 in Canberra. Single phase tops out at 9.5 kW, which is roughly 5 litres a minute in a Sydney winter. That is a basin, not a shower.
On 3-phase, it is a good swap. A 28 kW unit like the Stiebel Eltron DHE 27 ($1,545) or DEL 27 Plus ($1,399) gives about 13 to 17 litres a minute, enough for one good shower plus a tap. Smaller homes in warm climates can look at the DEL 18 Plus ($1,365). A licensed electrician runs a new dedicated circuit from the switchboard. Your old off-peak circuit is too small to reuse.
Prices current as of October 2026, AUD including GST, unit only. Installation not included.
No 3-phase? Do not pay for a supply upgrade just to fit an instant heater. It often costs more than the heater. Go gas continuous flow if you have mains gas, or a new storage tank or heat pump if you do not.
What it costs to convert storage to instantaneous hot water
Converting storage to instantaneous hot water costs the unit price plus installation, and the installation bill depends almost entirely on whether the gas line or electrical circuit is already adequate.
| Item | Gas tank to gas continuous flow | Electric tank to instant electric |
|---|---|---|
| Unit | $749 (M20) or $849 (M26) | $1,365 to $1,545 (19 to 28 kW, 3-phase) |
| Trades needed | Licensed gas fitter and plumber | Licensed electrician and plumber |
| Usual extra cost | Upsizing the gas pipe or meter | New dedicated 3-phase circuit |
| Old tank removal | Drain, disconnect, take away | Drain, disconnect, take away |
Unit prices current as of October 2026, AUD including GST. Installation is quoted by your installer and varies by site.
Get the installer to quote the whole job, removal and disposal included. A cheap unit price means nothing if the gas pipe needs replacing.
What changes once the tank is gone
The biggest change after removing a hot water tank is that hot water never runs out, though it still comes out at whatever flow rate the unit can heat.
- No more cold last shower. Back to back showers all get hot water. With a tank, the fourth teenager loses.
- Flow, not volume, is the limit. Two showers on a 20 litre unit in Melbourne in July will both run lukewarm. Size it for your worst winter morning.
- The wait at the tap stays the same. Cold water sitting in the pipe still has to clear first. Instant does not mean instant at the tap.
- Very low flows may not trigger the heater. Every instant unit has a minimum flow before it fires. A dribble from the kitchen mixer can stay cold.
- Space back. A 250 litre tank takes a cupboard. An instant unit is a box on the wall.
Running cost is not automatically lower. Gas continuous flow usually beats a gas tank, because there is no tank losing heat around the clock. Instant electric on a general tariff can cost more than an old electric tank heated on cheap off-peak power. Our breakdown of instant hot water vs storage tank covers that trade-off honestly.
Signs the old tank is due
An old hot water tank is due for replacement when it leaks from the base, gives rusty or smelly hot water, or is past about 10 to 15 years old.
A leak from the base of the tank is the end. The inner lining has gone and it will not be repaired. A dripping relief valve is different, that is often just the valve, so get it checked before you buy anything. Plan the swap before the tank fails. Choosing an instant system with cold showers and a flooded laundry leads to rushed decisions and whatever the plumber has on the van.
Not sure which way to go? Compare every unit in Instant Hot Water Systems or the Stiebel Eltron range.
Frequently asked questions
Can I replace my electric hot water tank with an instant system?
You can replace an electric hot water tank with a whole house instant electric system only if your home has 3-phase power, because single phase power is limited to about 9.5 kW, roughly 5 litres a minute in a Sydney winter. On single phase, the practical choices are gas continuous flow, a heat pump or another storage tank.
Can I replace a gas hot water tank with continuous flow in Australia?
You can replace a gas hot water tank with a gas continuous flow unit in Australia, and it is the simplest tank to instant swap, but a licensed gas fitter must confirm the gas meter and pipe can supply the larger burner. The unit also needs a standard power point for its controls.
How much does it cost to replace a hot water tank with instant in Australia?
Replacing a hot water tank with an instant system in Australia costs the unit plus installation, with gas continuous flow units from $749 and 3-phase instant electric units from $1,365 to $1,545 as of October 2026. Installation varies by site, and the biggest extra costs are upsizing a gas pipe or running a new 3-phase circuit.
Is instant hot water cheaper to run than a storage tank?
Gas continuous flow is usually cheaper to run than a gas storage tank because there is no tank losing heat all day, but instant electric on a general tariff can cost more than an electric tank heated on off-peak power. The answer depends on your fuel and tariff.
Do I need 3-phase power for instantaneous hot water?
You need 3-phase power for whole house electric instantaneous hot water, because one standard shower in winter needs about 15 to 21 kW depending on the city. Single phase units up to 9.5 kW suit a basin or small ensuite, and gas continuous flow needs no 3-phase at all.
Get the swap right first time
Tell us what your old tank is (gas or electric, and the litres on the label), your suburb, whether you have single or 3-phase power, and how many showers run at once. We will tell you which unit replaces it, or tell you plainly that instant is the wrong fit for your house.
Ask us what should replace your old tank →
This guide is general information for Australian households and is not a substitute for advice on your specific home and site. Electric instant hot water must be installed by a licensed electrician and plumber, and 3-phase units need a 3-phase supply. Gas continuous flow units must be installed by a licensed gas fitter and plumber, who will confirm gas supply capacity and flue requirements. Delivery temperature limits for bathrooms apply in every state; your plumber will set or fit what is required. Confirm final sizing and installation costs with us or your installer before you buy.